Spring Lake, a 68-acre lake located 3.5 miles northwest of Maple Valley in unincorporated King County, has infestations of four noxious aquatic weeds: Eurasian watermilfoil (Myriophyllum spicatum), purple loosestrife (Lythrum salicaria), yellow iris (Iris pseudacorus), and fragrant water lily (Nymphaea odorata). This project aims to eradicate these four species initially, and establish a community-based program of long-term monitoring and control of these aquatic noxious weeds. Eradication of Eurasian watermilfoil is of primary importance, as it threatens to deteriorate lake conditions if left untreated.
Control methods included aquatic herbicide applications, manual removal methods, and installation of bottom barrier material near the boat launch. These activities will be performed according to the approved Spring Lake Integrated Aquatic Vegetation Management Plan (IAVMP).
Spring Lake was treated with the liquid formulation of 2,4-D (DMA 4*IVM) on July 29, 2003 by a licensed applicator. (see treatment map) The main treatment area in the lake was a 14-acre shallow area (average depth about 1.5 feet) along the southwest and southern shoreline. Five other areas in the lake, totaling 10-acres, were treated. The largest of these five areas was a 5.5-acre area just north of the boat launch. Other areas were along the NW shore (1-acre), and the NE cove (2.5-acres), and two small areas along the eastern shore (totaling 1-acre). The shallow areas were treated at a rate of 2.1 gallons per acre and the deeper areas were treated at a rate of 7 gallons per acre. A total of 100 gallons was applied to the lake. A planned second 2,4-D treatment was not applied, based on the monitoring results.
Prior to treatment, five water samples were collected by King County staff for 2,4-D analysis, with no 2,4-D detected in any sample. The samples were collected at a depth of 1 meter using a Van Dorn sampler. Post-treatment samples were taken by King County staff at 1 meter depth from two stations in the lake. Two water samples were taken 5 days after treatment. The sample at Station 4 was reported to have 325 ppb (taken from the middle of the largest treatment area along the SW shore). The sample from mid-lake (Station 2, outside the treatment area, (.25 miles NE from the main treatment area) had a concentration of 405 ppb. Water samples were taken 16 days post treatment from Station 2 - 235 ppb; 35 days post-treatment (Station 2) 158 ppb; and 49 days post-treatment - 0.545ppb (Station 2). The samples were analyzed by Severn Trent Labs, Seattle, an Ecology-accredited laboratory using USEPA method 8151 GC/MS Modified. Compared to the 2,4-D results using the granular formulation, 2,4-D concentrations from the liquid formulation remained much higher than expected and persisted for a longer period of time. (see the figure below for the sampling locations)
During an August 18 snorkeling survey, King County staff reported no surviving milfoil. King County staff returned to the lake on September 29 to snorkel the lake a second time. There were no signs of new milfoil growth, or any surviving milfoil plants.
|The 2,4-D was effective in temporarily eradicating
the milfoil from the lake, assuming no milfoil survived below depths that
were visually surveyed. However, the 2,4-D levels remained well above the
irrigation standard (100 ppb) and the drinking water standard (70 ppb) for
much longer than anticipated. There were no unintended side effects of
elevated 2,4-D levels observed in the lake during either snorkel survey. A resident who watered with water from the lake just 6 days post-treatment
experienced minor damage to a royal fern (see photo). Although likely, the
damage was not confirmed to be herbicide related.
The applicator applied within label-recommended rates.
There was no milfoil detected in either the spring or the summer surveys in 2004!
King County divers hand pulled about 12 pounds fresh weight of milfoil from Spring Lake.
No milfoil has been detected in the spring and summer surveys.
|Time After Treatment|
|Inside Treatment Area||Outside Treatment Area (2)|
|5 days after treatment||325 ppb||405 ppb|
|16 days after treatment||no sample taken||235 ppb|
|35 days after treatment||no sample taken||158 ppb|
|49 days after treatment||no sample taken||.545 ppb|
Click here to see the sampling forms (word document)
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